Control system for machine having rotary cutting head

ABSTRACT

A control system may be provided for operating a machine having a rotary cutting head. The control system may include a wrist capable of rotating about a wrist axis, a gear mechanism capable of driving rotation of the wrist. A cutting head may be attached to the wrist and capable of rotating about a cutter axis that is substantially perpendicular to the wrist axis. The control system may also include primary and redundant angular sensors capable of generating signals indicative of angular orientations of the wrist. The control system may additionally include a controller, capable of controlling operation of the wrist and the cutting head, comparing an actual angular orientation of the wrist to a target angular orientation, as indicated by the primary and secondary signals, and selectively inhibiting the cutting head from operating when the actual angular orientation differs from the target angular orientation by an operation threshold.

TECHNICAL FIELD

The present disclosure relates generally to a machine having a rotarycutting head, and more particularly, to a system for controlling theposition of the rotary cutting head.

BACKGROUND

One manner of milling tunnels into rock is to repetitively pass acutting head across a vertical face of the rock. An overall efficiencyof a operation of this type may be governed by a cutting depth and aspeed of movement of the cutting head. These variables are oftenconstrained by the nature of the milled material and the orientation ofthe cutting head. For example, milling coal and hard rock often requiresdifferent techniques, cutting depths, speed of movements, and cuttinghead orientations.

Existing cutting methods may be inefficient when milling hard rock atcomparatively deep cutting depths because the cutting head is notmaintained in an optimal cutting orientation relative to the verticalface of the rock. Although this problem is not as prevalent when miningsoft materials, such as, for example, coal, the very nature of hard rockmay require specialized cutting heads and techniques.

Additionally, misalignment of the cutting head relative to the verticalrock face may damage a positioning member. Misalignment may damage thecutting head and other portions of a machine because of uneven feedbackand force transfer between the cutting head and the wrist. Such unevenfeedback may result in excessive wear and possibly significant damage tothe mining machine. Even relatively small misalignment of the cuttinghead may be detrimental when milling extremely hard rock.

One attempt to address these problems is disclosed in U.S. Pat. No.6,062,650 (“the '650 patent”). The '650 patent discloses an angularencoder for measuring the rotation angle of a turret (attached to acutting head) with reference to a predetermined line, usually a verticalaxis of the turret. The encoder sends signals to an onboard computer,which processes the signals according to a computer program. Thecomputer program assists a controller, which controls proportionalvalves, hydraulic cylinders, and the speed of rotation of the turret, tocut a preselected profile at a predetermined cut depth and rate ofadvance.

Although the encoder system of the '650 patent may help in cuttingpreselected milling profiles, the '650 patent may be prone to error asit may be possible for the encoder to fail without indication orwarning. This can lead to misalignment of the cutting head. Further, the'650 patent fails to account for potential misalignment of the cuttinghead. In particular, the control system of the '650 patent relies on theoutput of a single encoder without a reliable way to ensure that thecutting head remains in proper alignment relative to a rock face andthroughout extended use.

The machine, discussed herein, is directed at overcoming one or more ofthe problems set forth above and/or other problems in the prior art.

SUMMARY

One aspect of the present disclosure is directed to a control system fora machine. The control system may include a wrist capable of rotatingabout a wrist axis, a gear mechanism capable of driving rotation of thewrist, and a cutting head attached to the wrist and capable of rotatingabout a cutter axis that is substantially perpendicular to the wristaxis. The control system may additionally include a primary angularsensor capable of generating primary signals indicative of angularorientations of the wrist, and at least one redundant angular sensorcapable of generating secondary signals indicative of angularorientations of the wrist. The control system may further include acontroller capable of controlling operation of the wrist and the cuttinghead, and comparing an actual angular orientation of the wrist, asindicated by one or more of the primary signals or the secondarysignals, to a target angular orientation. The controller may alsoselectively inhibit the cutting head from operating when the controllerdetermines that the actual angular orientation differs from the targetangular orientation by an operation threshold.

Another aspect of the present disclosure is directed to a method foroperating a cutting head of a machine. The method may includedetermining a target angular orientation of the cutting head. The methodmay also include generating a primary angular orientation outputassociated with the cutting head, and generating a redundant angularorientation output associated with the cutting head. The method mayadditionally include making a first comparison of the primary angularorientation output to the target angular orientation, and selectivelyinhibiting the cutting head from operating based on the first comparisonwhen the primary angular orientation output and the target angularorientation differs by an operation threshold.

Another aspect of the present disclosure is directed to a machine. Themining machine may include a wrist capable of rotating about a wristaxis, a gear mechanism capable of driving rotation of the wrist andhaving an exposed surface with angular markings, and a stationarysurface adjacent the gear mechanism and having a reference marking. Themining machine may also include a cutting head attached to the wrist andcapable of rotating about a cutter axis that is substantiallyperpendicular to the wrist axis. The mining machine may additionallyinclude a primary encoder disposed in the gear mechanism and capable ofgenerating primary signals indicative of angular orientations of thewrist, and a redundant encoder disposed in the gear mechanism andcapable of generating secondary signals indicative of the angularorientations of the wrist. The mining machine may further include acontroller that is capable of controlling operation of the wrist and thecutting head, comparing an actual angular orientation of the wrist, asindicated by one or more of the primary signals or the secondarysignals, to a target angular orientation, and comparing the primarysignals to the secondary signals. The controller may also be capable ofselectively inhibiting the cutting head from operating when thecontroller determines that the actual angular orientation differs fromthe target angular orientation by an operation threshold, and when thecontroller determines that the primary signals differ from the secondarysignals by the operation threshold. Additionally, the controller may becapable of selectively causing a calibration instruction to be shown ona display when the controller determines the primary signals differ fromthe secondary signals by a signal threshold, and selectively causing awarning to be shown on the display when the controller determines theactual angular orientation differs from the target angular orientationby a warning threshold that is lower than the operation threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a machine;

FIG. 2 is a top plan view of the machine of FIG. 1;

FIG. 3 is a diagrammatic illustration of a portion of the machine ofFIGS. 1 and 2; and

FIG. 4 is an exemplary flowchart depicting a method of operating themachine of FIGS. 1 and 2.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, an exemplary machine 10 such as, forexample, a machine for creating tunnels, roadways, or shafts in material(such as rock, for example), is depicted. In this regard, machine 10 maybe any machine associated with various industrial applications,including, but not limited to, mining, agriculture, forestry,construction, and/or other industrial applications. Machine 10 mayinclude a cutting head 12 configured to mill the material by rotatingabout a cutter axis 13. Machine 10 may also include a frame 14 equippedwith ground engaging drive mechanisms, such as tracks 16. In oneembodiment, machine 10 may further include one or more motors that areoperatively connected to a remote power source (not shown), whichsupplies electric power to the machine via one or more tethered cables(not shown). Alternatively, machine 10 may have a localized powersource, such as an engine (not shown). The power source(s) may be usedto produce mechanical and electrical power outputs for driving andcontrolling the numerous components of machine 10.

Cutting tools 18 may be mounted on cutting head 12 such that cuttingtools 18 collectively rotate about cutter axis 13. Cutting tools 18 mayalso be capable of individually rotating a plurality of cutting edgesabout respective cutting tool axes (not shown) that are substantiallyperpendicular to cutter axis 13. Cutting head 12 and cutting tools 18may be moved to engage with a target cutting surface at any orientationby adjusting an angle of reproach of cutter axis 13. However, in orderto ensure efficient and damage free operation, cutting head 12 mayfollow a cutting plane aligned with cutter axis 13. That is, cuttinghead 12 should engage material in a direction perpendicular to the axialend face of cutting head 12. Additionally, when cutting head 12 ismoving along a particular milling path, the major axis (i.e., an amountequal to the diameter of cutting head 12) of cutting head 12 may bealigned perpendicular to the line of travel in order to mill the maximumamount of material. In at least one embodiment, a preferred cuttingorientation may be defined as an orientation in which cutter axis 13 issubstantially parallel to a target cutting surface, cutting edges ofcutting tools 18 are substantially perpendicular to the target cuttingsurface, and the major axis of cutting head 12 is substantiallyperpendicular to a target trajectory line of travel.

Wrist 19 may be configured to rotate cutting head 12 about a wrist axis20 to maintain cutting head 12 in a preferred cutting orientation.Positioning drum 22 may be configured to move wrist 19 vertically up anddown by pivoting about a lift axis 23. Boom 24 may be configured toswing cutting head 12, wrist 19, and positioning drum 22 about a swingaxis 25. Additionally, boom 24 may also be configured to extend andretract cutting head 12, wrist 19, and positioning drum 22 along anextension axis 26. Movement of boom 24 along extension axis 26 and swingaxis 25 may result in corresponding movements of positioning drum 22,wrist 19, and cutting head 12. Movement of positioning drum 22 alonglift axis 23 may result in corresponding movement of wrist 19 andcutting head 12. Rotational movement of wrist 19 may result inrotational movement of cutting head 12. Boom 24 may be configured tomove with the assistance of extension actuators 30 and swing actuators31. Positioning drum 22 may be configured to move with the assistance oflift actuators 32. Rotation of wrist 19 may be achieved by operation ofa gear mechanism 33. Gear mechanism 33 may be driven by a hydraulicmotor (not shown) or other similarly situated actuator.

As shown in FIG. 3, a controller 40 may be in communication withextension actuators 30, swing actuators 31, lift actuators 32, and gearmechanism 33. Controller 40 may be capable of causing actuators 30-32and gear mechanism 33 to move (e.g., electric or hydraulic circuitry).In this way, controller 40 may direct the rotating, swinging, lifting,pivoting, extending, and retracting movements of machine 10 andcomponents of machine 10. Controller 40 may be an electronic controllerthat operates in a logical fashion to perform operations, executecontrol algorithms, store and retrieve data and other desiredoperations. Controller 40 may include or access memory, secondarystorage devices, processors, and any other components for running anapplication. Various other circuits may be associated with controller 40such as power supply circuitry, signal conditioning circuitry, drivercircuitry, and other types of sensory circuitry.

The term “controller” is meant to be used in its broadest sense toinclude one or more controllers and/or microprocessors that may beassociated with the machine 10 and that may cooperate in controllingvarious functions and operations of machine 10. Controller 40 mayutilize one or more data maps relating to the kinematics of machine 10and the operating environment of which machine 10 may operate in.

Controller 40 may be in communication with a display interface 41, asshown in FIG. 3. In at least one embodiment, display interface 41 mayalso have an associated input device for communicating inputs that maybe stored on the memory of controller 40. Display interface 41 maydisplay information concerning the state of machine 10 (e.g., theangular orientation of cutting head 12). Controller 40 and displayinterface 41 may be located on machine 10 and/or may be distributed withcomponents located remotely off-board machine 10.

Machine 10 may be equipped with a plurality of sensors that provide dataindicative of various operating parameters of machine 10 and/or aspectsof the environment in which machine 10 may be operating in. The term“sensor” is meant to be used in its broadest sense to include one ormore sensors and related components associated with machine 10 thatcooperate to sense various functions, operations, and operatingcharacteristics.

In the example shown in FIG. 3, an environment sensor 43 and/or aposition sensor 46 may be included within a control system 47 that alsoincludes controller 40 and display interface 41. Sensors 43, 46 maysense a position and/or orientation (e.g., a heading, pitch, roll ortilt, and yaw) of machine 10. Controller 40 may process information fromsensors 43, 46 and use the information to control the motions of machine10. Sensors 43, 46 may take the form of numerous embodiments, some ofwhich will be explained now.

Environment sensor 43 may include one or more sensing elements thatinteract with the working environment of machine 10. For example,machine 10 may utilize lasers and prisms with the assistance of fieldengineering and surveying equipment. Based upon a known position of thelasers and reflected light from the prisms, a pose of machine 10, andmore particularly a pose of cutting head 12, may be determined. In otherembodiments, environment sensor 43 may be a RADAR sensor (radiodetection and ranging), a SONAR sensor (sound navigation and ranging), aLIDAR (light detection and ranging) sensor, and/or a camera visionsensor. Environment sensor 43 may generate data that is received bycontroller 40 and used to determine the position and orientation ofcutting surfaces shown in FIGS. 1 and 2. Such surfaces may include afloor face 50, a lower cutting face 52, upper cutting face 53, and aceiling face 54.

Position sensor 46 may include one or more sensing elements thatcooperate to generate and provide an actual position and/or orientationof machine 10 to controller 40. In some embodiments, position sensor 46may detect the actual position and/or orientation of cutting head 12relative to a reference location by utilizing a database of mapsconsisting of known kinematics of machine 10. In other embodiments,position sensor 46 may include a slope sensing element, an inclinationsensing element, a pitch angle sensing element, an accelerometer, astrain gauge, etc. Position sensor 46 may be disposed on or adjacent tocutting head 12 for improved accuracy. For example, the sensing elementsmay be associated with gear mechanism 33 for directly detecting anangular orientation of cutting head 12 relative to frame 14.

FIG. 3, in addition to showing control system 47, also illustrates aninternal cross-section of gear mechanism 33. It should be noted thatFIG. 3 is exemplary in nature and is not drawn to scale, and certainparts may be removed and/or exaggerated for ease of understanding. Asshown in FIG. 3, gear mechanism 33 may include a stationary platform 58on which a plurality of pinion gears 60 are rotatably supported. Piniongears 60 may be capable of rotating about pinion gear axes 61. Piniongears 60 may be driven, for example, by independent hydraulic motors(not shown) located behind platform 58, and/or by a common gear (e.g., asun gear). Pinion gears 60 may engage a common ring gear 62, such thatrotation of pinion gears 60 about their respective pinion gear axes 61translates into rotation of ring gear 62 about axis 20. Cutting head 12may be operatively connected to ring gear 62, such that a rotation ofring gear 62 results in a corresponding rotation of cutting head 12.

Control system 47 may further include a primary angular sensor 63 and aredundant angular sensor 64 associated with gear mechanism 33. Althoughthe exemplary control system 47 is shown with two sensors 63, 64 it iscontemplated that some embodiments may have more than two sensors 63,64. For example, a primary angular sensor 63 and at least one redundantangular sensor 64. Sensors 63, 64 may be configured to detect an angularorientation of wrist 19, (e.g., relative to platform 58 and/or frame 14)by detecting a rotation of ring gear 62. Sensors 63, 64 may be optical,magnetic, capacitive, or geared sensors. For example, sensors 63, 64 maybe rotary encoders that convert the angular position of ring gear 62 toan analog or digital signal for use by controller 40. The signals mayindicate an angular orientation of wrist 19 and cutting head 12.

As encoders, sensors 63, 64 may engage ring gear 62 via gear teeth inthe same way that pinion gears 60 engage ring gear 62. Sensors 63 and 64may also include a sensing element that detects when each tooth, or aparticular tooth, passes by the sensing element (e.g., an opticalsensor). Each time a tooth movement is detected a signal associated withthe detection may be generated. Alternatively, a sensing element coulddetect a portion of a shaft connected to the gear teeth. Other methodsmay be available as would be understood by a person having ordinaryskill in the art.

In some embodiments, a portion of gear mechanism 33 (e.g., an axial endface of ring gear 62) may have an exposed surface along its perimeterthat is marked with a plurality of angular markings 65. Angular markings65 may be evenly spaced around the circumference or perimeter of gearmechanism 33. For example, angular markings 65 may be evenly distributedaround the circumference at every 2 degrees. A reference marking 66 (SeeFIG. 1) may be provided along a non-rotatable surface adjacent to gearmechanism 33. For example, reference marking 66 may be located onpositioning drum 22. Reference marking 66 may be configured to alignwith angular markings 65 to mechanically indicate an angular orientationof wrist 19. Angular markings 65 and reference marking 66 may beviewable from a ground position or any other external vantage location.

FIG. 4 is an exemplary flowchart depicting a positioning method that maybe implemented by controller 40 during operation of the mining machine10 of FIGS. 1 and 2.

FIG. 4 will be disclosed in more detail in the next section to furtherillustrate the disclosed concepts.

INDUSTRIAL APPLICABILITY

The disclosed control system may be applicable to any machine having arotatable cutting head, and more particularly applicable to machineshaving rotatable cutting heads that move three dimensionally and mustmaintain angular alignment in relation to a dynamically modifiedsurface. The disclosed control system may improve efficiency and preventunnecessary wear and tear of cutting head 12 by maintaining cutting head12 in a preferred orientation relative to a target cutting surface.

FIG. 4 is an exemplary flowchart illustrating operation of controlsystem 47, which will now be explained with reference to components ofFIGS. 1-3. When performing an operation, such as an operation controller40 may determine a target angular orientation of cutting head 12 (Step100).

The target angular orientation may be a preferred cutting orientationthat allows material to be milled during engagement with cutting faces52 and 53. In at least one embodiment, the target angular orientationmay be defined as a wrist 19 orientation that maintains the cutter axis13 of cutting head 12 substantially parallel to a cutting face 52 duringmovement in a direction aligned with cutter axis 13. In other words anaxial end face of cutting head 12 should be generally perpendicular tothe trajectory of cutting head 12 during milling. Additionally, thetarget angular orientation may dynamically change throughout operationof a milling process, based on a change in the trajectory, and may becontinuously updated during operation. Determining the target angularorientation may consist of establishing a wrist angle that ranges from 0degrees to 360 degrees for a particular cutting segment.

Controller 40 may be in communication with a plurality of sensors (e.g,environment sensor 43, position sensor 46, and sensors 63, 64) andreceive signals from these sensors (Step 102). These signals may includea primary signal generated by primary angular sensor 63, a secondarysignal generated by redundant angular sensor 64, and an environmentsignal generated by environment sensor 43 and/or position sensor 46.

Controller 40 may then determine whether the primary signal is available(Step 104). If the primary signal is unavailable (e.g., when primaryangular sensor 63 has failed or when the signal from primary angularsensor 63 is out of range), controller 40 may cause display interface 41to show a warning, and continue operation by using only the secondarysignals generated by redundant angular sensor 64 (Step 106). The warningmay be a graphic image, textual indication, and may appropriatelyindicate whether the primary angular sensor 63 has failed or if thesignals from primary angular sensor 63 are out of range. As long ascontroller 40 determines that the primary signal is available (Step 104:Yes), controller 40 may then compare the primary signals to thesecondary signals. That is, controller 40 may determine if a differencebetween the primary and redundant sensors 63, 64 is greater than anoperation threshold and/or a signal comparison threshold (Step 108). Forexample, controller 40 may process the primary signals into a primaryvalue and process the secondary signals into a secondary value (e.g.,each ranging from 0 degrees to 360 degrees). Controller 40 may thencalculate a difference between the primary value and the secondary valueand determine if the difference is greater than an operation thresholdamount and/or a signal comparison threshold. When the difference isgreater than the operation threshold (Step 108: Yes) controller 40 mayinhibit milling of cutting head 12 (Step 116). When the difference isgreater than the signal comparison threshold (Step 108: Yes) controller40 may cause display interface device 41 to show an instruction tocalibrate sensors 63, 64 (Step 110). When the difference is less thanthe signal comparison threshold (Step 108: No) controller 40 maycontinue operation by using only the primary signals (Step 112).

Controller 40 may perform a comparison of the actual angular orientationof wrist 19 to the target angular orientation of wrist 19 (Step 114).The actual angular orientation may be established by determining anactual angular orientation value based on the primary or secondarysignals, depending on completion of step 106 or step 112. The actualangular orientation value may range from 0 degrees to 360 degrees.Controller 40 may also use the environment signals to determine thetarget angular orientation of wrist 19. For example, controller 40 mayuse information received from environment sensor 43 to dynamically map asurface plane of cutting face 52 and/or 53. Controller 40 may thendetermine the target angular orientation based on the map of the surfaceplane and the preferred orientation requirements for efficient anddamage free milling. Controller 40 may then compare a difference betweenthe actual angular orientation and target angular orientation to anoperation threshold.

If the difference between the actual angular orientation and targetangular orientations is greater than an operation threshold (Step 114:Yes), controller 40 may inhibit milling of cutting head 12 (Step 116).Inhibiting milling may consist of powering down cutting head 12 andcutting tools 18 or it may consist of decelerating the movement ofcutting head 12 and cutting tools 18. Operation of machine 10 may remaininhibited until the difference between the actual angular orientationand target angular orientations is less than the operation threshold. Insome instances re-orientating of boom 24, positioning drum 22, and/orwrist 19 may be required. If the difference determined at step 114 isless than the operation threshold (Step 114: No), controller 40 may thencompare the difference to a lower warning threshold (Step 118). If thedifference is greater than the lower warning threshold (Step 118: Yes),controller 40 may cause display interface 41 to show a warning (Step120). Controller 40 may allow milling to occur following steps 118 and120 (Step 122).

In some embodiments, a machine operator may be able to visually verifythe actual angular orientation of wrist 19 by comparing alignment ofangular marking 65 with reference marking 66. Additionally, the machineoperator may use angular marking 65 and reference marking 66 tocalibrate or re-calibrate primary angular sensor 63 and/or redundantangular sensor 64. In order to do this, the machine operator mayincrementally rotate wrist 19 about wrist axis 20 until angular marking65 aligns with reference marking 66. Then the machine operator may usedisplay interface 41 to electronically zero out the signals from sensors63 and 64.

In some embodiments, a machine operator may perform manual movement ofwrist 19 in order to determine if a specific sensor has failed and/or isout of range. For example, controller 40 may cause display interface 41to display a calibration instruction (Step: 110) and further prompt themachine operator to select a specific sensor to disable and/or aspecific sensor to rely on. In other embodiments, control system 40 mayexecute an intelligent calibration that is performed automatically. Forexample, when controller 40 determines that the difference betweenprimary and secondary signals is greater than the operation threshold(Step 108: Yes) controller 40 may inhibit milling of cutting head 12(Step 116) and execute an automatic intelligent calibration. Theautomatic intelligent calibration may rotate wrist 19 counter-clockwiseand verify that sensors 63,64 are generating signal outputs that areexpected for the degree of counter-clockwise rotation (i.e., compare thesignal output values to the expected output values). The automaticintelligent calibration may additionally rotate wrist 19 clockwise andverify that sensors 63,64 are generating signal outputs as expected forthe degree of clockwise rotation (i.e., compare the signal output valuesto the expected output values). If Controller 40 determines that thedifference between the signal outputs and expected outputs is greaterthan an expectation threshold controller 40 may cause a specific sensorwarning to be shown on display interface 41.

Several benefits may be associated with the disclosed control system.Specifically, the disclosed control system may reduce unnecessary wearand tear by maintaining cutting head 12 in a preferred orientationrelative to a target cutting surface during an operation. By maintainingcutting head 12 in the preferred orientation, the longevity andproduction capability of machine 10 may be increased. Additionally, thedisclosed control system may inhibit operation when cutting head 12 issignificantly misaligned, thereby avoiding catastrophic damage.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the disclosed controlsystem. Other embodiments will be apparent to those skilled in the artfrom consideration of the specification and practice of the disclosedcontrol system. It is intended that the specification and examples beconsidered as exemplary only, with a true scope being indicated by thefollowing claims and their equivalents. As used herein, the articles “a”and “an” are intended to include one or more items, and may be usedinterchangeably with “one or more.” Where only one item is intended, theterm “one” or similar language is used. Also, as used herein, the terms“has,” “have,” “having,” or the like are intended to be open-endedterms. Further, the phrase “based on” is intended to mean “based, atleast in part, on” unless explicitly stated otherwise.

What is claimed is:
 1. A control system for a machine, the controlsystem comprising: a wrist configured to rotate about a wrist axis; agear mechanism configured to drive rotation of the wrist; a cutting headcoupled to the wrist and configured to rotate about a cutter axis thatis substantially perpendicular to the wrist axis; a primary angularsensor configured to generate primary signals indicative of angularorientations of the wrist; at least one redundant angular sensorconfigured to generate secondary signals indicative of angularorientations of the wrist; and a controller configured to: controloperation of the wrist and the cutting head; compare an actual angularorientation of the wrist, as indicated by one or more of the primarysignals or the secondary signals, to a target angular orientation; andselectively inhibit the cutting head from operating when the controllerdetermines that the actual angular orientation differs from the targetangular orientation by an operation threshold.
 2. The control system ofclaim 1, wherein the controller is further configured to controloperation of the wrist based on only one or more of the primary signals.3. The control system of claim 1, wherein the controller is furtherconfigured to control operation of the wrist based on only the secondarysignals when the primary signals are unavailable and when the primarysignals are out of range.
 4. The control system of claim 1, furtherincluding a display in communication with the controller, wherein thecontroller is configured to: compare the primary signals and thesecondary signals to an expectation value, and selectively cause aspecific sensor warning to be shown on the display when the controllerdetermines that at least one of the primary signals or the secondarysignals differs from the expectation value by an expectation threshold.5. The control system of claim 1, further including a display incommunication with the controller, wherein the controller is configuredto selectively cause a signal warning to be shown on the display whenthe controller determines the primary signals are unavailable and whenthe controller determines the primary signals are out of range.
 6. Thecontrol system of claim 1, further including a display in communicationwith the controller, wherein the controller is configured to: comparethe primary signals to the secondary signals; selectively inhibit thecutting head from operating when the controller determines that theprimary signals differ from the secondary signals by the operationthreshold; and selectively cause a calibration instruction to be shownon the display when the controller determines that the primary signalsdiffer from the secondary signals by a signal threshold.
 7. The controlsystem of claim 1, further including a display in communication with thecontroller, wherein the controller is configured to selectively cause awarning to be shown on the display when the controller determines thatthe actual angular orientation differs from the target angularorientation by a warning threshold that is lower than the operationthreshold.
 8. The control system of claim 1, further including anenvironment sensor in communication with the controller and configuredto generate environment signals, wherein the controller is furtherconfigured to: compare an actual angular orientation of the wrist, asprovided via the environment signals, to the target angular orientation;and selectively inhibit the cutting head from operating when thecontroller determines that the actual angular orientation differs fromthe target angular orientation by the operation threshold.
 9. Thecontrol system of claim 1, wherein the target angular orientation is apreferred orientation in which an axial end face of the cutting head isperpendicular to an operation milling direction.
 10. A method foroperating a cutting head of a machine, the method comprising:determining a target angular orientation of the cutting head; generatinga primary angular orientation output associated with the cutting head;generating a redundant angular orientation output associated with thecutting head; making a first comparison of the primary angularorientation output to the target angular orientation; and selectivelyinhibiting the cutting head from operating based on the first comparisonwhen the primary angular orientation output and the target angularorientation differ by an operation threshold.
 11. The method of claim10, further including: making a second comparison of the redundantangular orientation output to the target angular orientation only whenthe primary angular orientation output is unavailable and when theprimary angular orientation output is out of range; and selectivelyinhibiting the cutting head from operating based on the secondcomparison when the redundant angular orientation output and the targetangular orientation differs by the operation threshold.
 12. The methodof claim 11, further including displaying a second warning when theredundant angular orientation output and the target angular orientationdiffers by a warning threshold that is lower than the operationthreshold.
 13. The method of claim 10, further including: generating anenvironment angular orientation output associated with the cutting head;making a third comparison of the environment angular orientation outputto the target angular orientation; and selectively inhibiting thecutting head from operating based on the third comparison when theenvironment angular orientation output and the target angularorientation differs by the operation threshold.
 14. The method of claim10, further including displaying a signal warning when the primaryorientation output is unavailable and when the primary orientationoutput is out of range.
 15. The method of claim 10, further including:comparing the primary angular orientation output to the redundantangular orientation output; selectively inhibiting the cutting head fromoperating based on the third comparison when the environment angularorientation output and the target angular orientation differs by theoperation threshold; and displaying a calibration instruction when theprimary orientation output and the redundant orientation output differsby a signal threshold.
 16. The method of claim 10, further including:comparing the primary angular orientation output to the redundantangular orientation output; and displaying a second warning when theprimary angular orientation output and the target angular orientationdiffers by a warning threshold that is lower than the operationthreshold
 17. A machine, comprising: a wrist configured to rotate abouta wrist axis; a gear mechanism configured to drive rotation of the wristand having an exposed surface with a plurality of angular markings; astationary surface adjacent the gear mechanism and having a referencemarking; a cutting head coupled to the wrist and configured to rotateabout a cutter axis that is substantially perpendicular to the wristaxis; a primary encoder disposed in the gear mechanism and configured togenerate primary signals indicative of angular orientations of thewrist; at least one redundant encoder disposed in the gear mechanism andconfigured to generate secondary signals indicative of the angularorientations of the wrist; and a controller configured to: controloperation of the wrist and the cutting head; compare an actual angularorientation of the wrist, as indicated by one or more of the primarysignals or the secondary signals, to a target angular orientation;compare the primary signals to the secondary signals; selectivelyinhibit the cutting head from operating when the controller determinesthat the actual angular orientation differs from the target angularorientation by an operation threshold and when the controller determinesthat the primary signals differ from the secondary signals by theoperation threshold; selectively cause a calibration instruction to beshown on a display when the controller determines the primary signalsdiffer from the secondary signals by a signal threshold; and selectivelycause a warning to be shown on the display when the controllerdetermines the actual angular orientation differs from the targetangular orientation by a warning threshold that is lower than theoperation threshold.
 18. The control system of claim 17, wherein thecontroller is further configured to control operation of the wrist basedon only the primary signals as long as the primary signals are availableand within range.
 19. The control system of claim 18, wherein thecontroller is further configured to control operation of the wrist basedon only the secondary signals when the controller determines the primarysignals are unavailable and when the controller determines the primarysignals are out of range.
 20. The control system of claim 17, furtherincluding an environment sensor in communication with the controller andconfigured to generate environment signals, wherein the controller isfurther configured to: compare an actual angular orientation of thewrist, as provided via the environment signals, to the target angularorientation; and selectively inhibit the cutting head from operatingwhen the controller determines that the actual angular orientationdiffers from the target angular orientation by the operation threshold.